The invention relates to a device for connecting casings.
Oil and gas wells are lined by setting a pipe called a casing which is concreted into position through the supply of a cement slurry to the annulus outside the casing.
The casing is formed by interconnecting shorter pipes screwed one to the other by means of threaded connections. A new pipe is supplied to the top of the casing which, then, is lowered one pipe length down into the well. Before a new pipe can be added to the top of the casing, the latter is refilled with a liquid.
Pipes joined together to form a coherent casing are, at each end thereof, provided with an external threaded portion, and pipes are added to each other by means of an external socket having internal threads. Before a new pipe is added to the top of the casing, a socket is screwed halfway onto the threaded portion at the upper end of the pipe, so that the socket forms a small extension of the pipe. The new pipe is suspended from a collar, called an elevator, which is closed around the pipe below the socket. The pipe suspended from the socket can be rotated in the elevator.
The elevator is assigned a vigorous drawworks for vertical positioning. The pipe is lowered by means of the drawworks until threads at the lower end of the pipe come into contact with the complimentary threads in a socket at the top of the casing. A tool, a spinner, grips and surrounds the pipe and rotates it about its own axis in order to screw the pipe into the socket at the top of the casing. Then, the threaded connection is tightened to the moment prescribed by means of a moment tool. The casing is lowered, suspended from the socket at the upper end of the latest added pipe, down into the well, and the process is repeated, the socket at the upper end of the latest added pipe, now located at the upper end of the casing, being ready to accommodate the end of a new pipe.
The described prior art technique suffers from a plurality of disadvantages. The drawworks is a rough and coarse apparatus having a lifting capacity of several hundred tons. It is difficult to fine adjust the vertical position, and the threads are readily damaged when a new pipe is lowered towards the top of the casing. As the pipe is screwed into the top of the casing, it is necessary to readjust the position of the elevator, i.e. to lower the elevator. In order to avoid that the threads are rubbed or become damaged in another way, the pipe should be lowered such that there does not arise any significant axial force acting on the threads while the pipe is rotated and screwed to the top of the casing. As mentioned, this is difficult to achieve by means of the drawworks.
The spinner used to screw a new pipe to the top of the casing, is expensive. Also, it is very space-demanding, as it must be capable of surrounding the largest casings in use.
Filling liquid into the casing involves liquid spillage, making the drill floor slippery and dangerous, thus, entailing the need for constant washing down and cleaning.